support serialization of classes (#17856)
Summary:
Stack:
:black_circle: **#17856 [jit] support serialization of classes** [:yellow_heart:](https://our.intern.facebook.com/intern/diff/D14402599/)
Add support for saving/loading TorchScript modules that depend on user-defned classes.
We track class dependencies the same we track tensor constants, then write them
all out such that we can just compile them in order before compiling the module
hierarchy.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17856
Reviewed By: shannonzhu
Differential Revision: D14461599
Pulled By: suo
fbshipit-source-id: 7115f87e069fd00dc8381d7de9997864fef7ea9f
